sweat of one's brow

A reference to Genesis 3:17 and 19 in the Bible (King James Version; spelling modernized): “And unto Adam he [God] said, because thou hast hearkened unto the voice of thy wife, and hast eaten of the tree, [...] In the sweat of thy face shalt thou eat bread, till thou return unto the ground: [...]”, meaning that due to disobedience to God, humans would henceforth have to exert labour for their food, causing their brows or foreheads to perspire.